Sunday, December 21, 2014

Beneath the Twin Cities, sandwiched snugly between the crumbling off-white edifice of the St. Peter Sandstone and the proudly jutting gray to tan Platteville Formation, is a gray to brown layer about 3 to 5 feet thick (or about a meter to a meter and a half for our metric friends), composed primarily of shale. This is the Glenwood Formation, sometimes known as the Glenwood Shale (which is a bit of misnomer because of the sand content). Like a fawn or baby bird, it hides from the eye, although we know it must be there, because heavens knows where else we would get adult deer and birds, and the Platteville has yet to collapse onto the St. Peter. The only reliable way to find it is to look for places where both the Platteville and St. Peter are exposed, and then check the area between them. Even then, you might not be able to see it well; for example, at Minnehaha Falls, the Glenwood is so deeply recessed you can't see much at all. The most obvious exposures are at Lock & Dam 1 and Dayton's Bluff.

Friday, December 12, 2014

Technicalities. Say you've got this thing, it's got two halves to its shell, the halves are called "valves", "bi" is two, "bivalve". Simple, right? Not so fast! There are bivalves, in the simple "it's got two half shells" sense, and then there are bivalves, in the "mollusk that belongs to Class Bivalvia" sense. Why bring this up? Aside from bivalved mollusks, we've also got another group of invertebrates with two half shells that is quite common in the fossil record, with abundant examples in the Decorah and Platteville formations. This group is Brachiopoda.